Active matrix type liquid crystal display

ABSTRACT

Disclosed is an in-plane switching active matrix type liquid crystal display with greater improvements on color shifting and contrast. The liquid crystal display comprises an in-plane switching type liquid crystal display panel having an active device substrate, an opposing substrate and a liquid crystal layer held sandwiched between the active device substrate and the opposing substrate, a first polarizer laid out on one side of the liquid crystal display panel, a second polarizer laid out on the opposite side of the liquid crystal display panel, first to third optical compensators placed in order between the liquid crystal display panel and the first polarizer, and a fourth optical compensator placed between the liquid crystal display panel and the second polarizer. As the first to fourth optical compensators are provided to compensate for retardation of the liquid crystal layer and retardation of the polarizers, black stretching does not occur even when observation is made from any viewing angle, and a reduction in contrast does not occur. Nor does color shifting occur at the time of displaying black.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an in-plane switching (IPS) activematrix type liquid crystal display, and, more particularly, to a liquidcrystal display which suppresses a reduction in contrast and colorshifting when the viewing angle changes.

2. Description of the Related Art

An IPS liquid crystal display presents image display by forming anelectric field, parallel to a liquid crystal substrate, between pixelelectrodes and a common electrode, and has an advantage of providing awider viewing angle over the TN mode type or the like, which forms anelectric field perpendicular to the substrate. FIG. 1 shows thestructure of a conventional IPS active matrix type liquid crystaldisplay. As shown in FIG. a1, the liquid crystal display has a liquidcrystal display (LCD) panel 10, a first polarizer 31 laid out on the topside of the LCD panel 10 and a second polarizer 32 laid out on thebottom side of the LCD panel 10. The LCD panel 10 comprises an activedevice substrate 11 on which scan lines 111, data lines 112, thin filmtransistors (TFTs) 113, pixel electrodes 114, a common electrode 115, acommon electrode line 116, etc. are formed, an opposing substrate 12 onwhich a black matrix 121, color layers (color filters) 122, etc. areformed, and a liquid crystal layer 13 held sandwiched between the activedevice substrate 11 and the opposing substrate 12. As shown in FIG. 2Awhich is an exemplary cross-sectional view of the conventional IPSactive matrix type liquid crystal display and FIG. 2B which shows thealignment direction of the liquid crystal layer 13 and the directions ofadsorption axes of the first and second polarizers 31 and 32, thedirection of the adsorption axis of the first polarizer 31 is setperpendicular to the alignment direction of the liquid crystal layer 13and the direction of the adsorption axis of the second polarizer 32 isset parallel to the alignment direction of the liquid crystal layer 13.

According to the conventional IPS active matrix type liquid crystaldisplay, the liquid crystal layer 13 has a birefringence if the viewingangle is changed even when no electric field is applied to the liquidcrystal layer 13, so that the adsorption axes of the polarizers 31 and32 do not appear to perpendicularly cross each other as observation ismade obliquely. That is, the LCD panel 10 in a black display statecauses a birefringence effect due to the apparent deviation between thealignment direction and the polarization plane that is caused by obliqueobservation of the liquid crystal layer 13. In case of obliqueobservation, the birefringence of the protection layer of the polarizerinfluences polarized light that passes the liquid crystal display. Thepolarizer comprises a polarization layer formed of a material having apolarization property and a protection layer which protects thepolarization layer. It is known that triacetyl cellulose which isgenerally used to form the protection layer has an optical anisotropyduring the fabrication process of the polarizer. The optical anisotropycauses birefringence with respect to light which passes the liquidcrystal display at the time the viewing angle of the liquid crystaldisplay is changed, thereby degrading the viewing angle characteristic.Such degradation increases the luminance in a dark state in case ofconducting oblique observation, thus lowering the contrast. FIG. 3Ashows the results of actually measuring the viewing angle characteristicfor the contrast of the conventional liquid crystal display. As seenfrom the diagram, there is an area with a contrast of less than 5 asobservation is made obliquely.

In case where oblique observation is made, the optical path becomeslonger as will be discussed later with reference to FIG. 5B, so that theapparent retardation of the liquid crystal layer changes. When theviewing angle is changed, therefore, the wavelength of light whichpasses the liquid crystal display varies so that the colors on thescreen look changed, thus causing so-called color shifting that dependson the direction of observation. FIG. 3B shows the results of measuringthe viewing angle characteristic for the chromaticity of theconventional liquid crystal display with the conventional structure. Asapparent from the diagram, the chromaticity varies significantly with achange in viewing angle. FIGS. 3A and 3B respectively correspond toFIGS. 6A and 6B.

Various schemes have been proposed to prevent a reduction in contrastand color shifting that depend on the viewing angle of such aconventional IPS type liquid crystal display. For example, JapanesePatent Laid-Open No. 133408/1999 has proposed a scheme of intervening acompensation layer having an optical anisotropy between a liquid crystallayer and a pair of polarizers which sandwich the liquid crystal layer.While this scheme effectively works on color shifting, however, thepublication fails to mention that the scheme improves the contrast.Japanese Patent Laid-Open No. 2001-242462 has proposed a scheme ofintervening first and second retardation plates between a liquid crystallayer and a pair of polarizers which sandwich the liquid crystal layer.Although the publication describes that the scheme effectively improvescolor shifting and the contrast, higher improvements are desirable.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an in-planeswitching active matrix type liquid crystal display with greaterimprovements on color shifting and contrast, as compared with the priorart.

An active matrix type liquid crystal display according to the inventioncomprises an in-plane switching type liquid crystal display panel havingan active device substrate, an opposing substrate and a liquid crystallayer held sandwiched between the active device substrate and theopposing substrate; a first polarizer laid out on one side of the liquidcrystal display panel; a second polarizer laid out on the opposite sideof the liquid crystal display panel; an optical compensator, placedbetween the first polarizer and the second polarizer, for compensatingfor retardation of the liquid crystal layer; and another opticalcompensator, placed between the first polarizer and the secondpolarizer, for compensating for retardation of the first or secondpolarizer.

In the liquid crystal display, each of the optical compensators may becomprised of a single optical compensator or a plurality of opticalcompensators. Each of the optical compensators may be located eitherbetween the liquid crystal display panel and the first polarizer orbetween the liquid crystal display panel and the second polarizer, orboth. In this case, each of absorption axes of the first and secondpolarizers may be set parallel to or perpendicular to an alignmentdirection of the liquid crystal layer and a direction of a refractiveindex nx of each of the optical compensators may be set parallel to orperpendicular to the alignment direction of the liquid crystal layer.

According to the first mode of the invention, the first polarizer islaid out on an opposing substrate side of the liquid crystal displaypanel, first to third optical compensators are laid out in order betweenthe liquid crystal display panel and the first polarizer from a liquidcrystal display panel side, a fourth optical compensator is laid outbetween the liquid crystal display panel and the second polarizer, adirection of each of refractive indexes nx of the first to third opticalcompensators is set parallel to or perpendicular to an alignmentdirection of the liquid crystal layer, and a direction of a refractiveindex nx of the fourth optical compensator is set parallel to orperpendicular to the alignment direction of the liquid crystal layer.

In this mode, an absorption axis of the first polarizer may be setperpendicular to the alignment direction of the liquid crystal layer andan absorption axis of the second polarizer may be set parallel to thealignment direction of the liquid crystal layer. The direction of therefractive index nx of the first optical compensator may be set parallelto the alignment direction of the liquid crystal layer, the direction ofthe refractive index nx of the second optical compensator may be setperpendicular to the alignment direction of the liquid crystal layer,the direction of the refractive index nx of the third opticalcompensator may be set parallel to a direction of the absorption axis ofthe first polarizer and the direction of the refractive index nx of thefourth optical compensator may be set parallel to a direction of theabsorption axis of the second polarizer.

According to the second mode of the invention, the first polarizer islaid out on an opposing substrate side of the liquid crystal displaypanel, first and second optical compensators are laid out in orderbetween the liquid crystal display panel and the first polarizer from aliquid crystal display panel side, third and fourth optical compensatorsare laid out in order between the liquid crystal display panel and thesecond polarizer from the liquid crystal display panel side, a directionof each of refractive indexes nx of the first and second opticalcompensators is set parallel to or perpendicular to an alignmentdirection of the liquid crystal layer, and a direction of each ofrefractive indexes nx of the third and fourth optical compensators isset parallel to or perpendicular to the alignment direction of theliquid crystal layer.

In this mode, an absorption axis of the first polarizer may be setperpendicular to the alignment direction of the liquid crystal layer andan absorption axis of the second polarizer may be set parallel to thealignment direction of the liquid crystal layer. The direction of therefractive index nx of the first optical compensator may be setperpendicular to the alignment . . . direction of the liquid crystallayer, the direction of the refractive index nx of the second opticalcompensator may be set parallel to a direction of the absorption axis ofthe first polarizer, the direction of the refractive index nx of thethird optical compensator may be set parallel to the alignment directionof the liquid crystal layer, and the direction of the refractive indexnx of the fourth optical compensator may be set parallel to a directionof the absorption axis of the second polarizer.

According to the third mode of the invention, the first polarizer islaid out on an opposing substrate side of the liquid crystal displaypanel, first and second optical compensators are laid out in orderbetween the liquid crystal display panel and the first polarizer from aliquid crystal display panel side, and a direction of each of refractiveindexes nx of the first and second optical compensators is set parallelto or perpendicular to an alignment direction of the liquid crystallayer.

In this mode, an absorption axis of the first polarizer may be setperpendicular to the alignment direction of the liquid crystal layer andan absorption axis of the second polarizer may be set parallel to thealignment direction of the liquid crystal layer. The direction of therefractive index nx of the first optical compensator may be set parallelto the alignment direction of the liquid crystal layer, and thedirection of the refractive index nx of the second optical compensatormay be set perpendicular to the alignment direction of the liquidcrystal layer.

According to a modification of the third mode of the invention, thesecond polarizer is laid out on an active device substrate side of theliquid crystal display panel, first and second optical compensators arelaid out in order between the liquid crystal display panel and thesecond polarizer from a liquid crystal display panel side, and adirection of each of refractive indexes nx of the first and secondoptical compensators is set parallel to or perpendicular to an alignmentdirection of the liquid crystal layer.

In the modification, an absorption axis of the first polarizer may beset parallel to the alignment direction of the liquid crystal layer andan absorption axis of the second polarizer may be set perpendicular tothe alignment direction of the liquid crystal layer. The direction ofthe refractive index nx of the first optical compensator may be setparallel to the alignment direction of the liquid crystal layer, and thedirection of the refractive index nx of the second optical compensatormay be set perpendicular to the alignment direction of the liquidcrystal layer.

According to the fourth mode of the invention, the first polarizer islaid out on an opposing substrate side of the liquid crystal displaypanel, a first optical compensator is laid out between the liquidcrystal display panel and the first polarizer, a second opticalcompensator is laid out between the liquid crystal display panel and thesecond polarizer, and a direction of each of refractive indexes nx ofthe first and second optical compensators is set parallel to orperpendicular to an alignment direction of the liquid crystal layer.

In this mode, an absorption axis of the first polarizer may be setperpendicular to the alignment direction of the liquid crystal layer andan absorption axis of the second polarizer may be set parallel to thealignment direction of the liquid crystal layer. The direction of therefractive index nx of the first optical compensator may be setperpendicular to the alignment direction of the liquid crystal layer,and the direction of the refractive index nx of the second opticalcompensator may be set parallel to the alignment direction of the liquidcrystal layer.

As the liquid crystal display according to the invention is provide withthe optical compensator which compensates for retardation of the liquidcrystal layer and the optical compensator which compensates forretardation of the first or second polarizer, black stretching does notoccur even when observation is made from any viewing angle, and areduction in contrast does not occur. Nor does color shifting occur atthe time of displaying black.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the schematic structure of aconventional IPS active matrix type liquid crystal display;

FIGS. 2A and 2B are diagrams exemplarily illustrating the conventionalliquid crystal display;

FIGS. 3A and 3B are viewing angle characteristic charts for the contrastand chromaticity according to the conventional liquid crystal display;

FIGS. 4A and 4B are diagrams exemplarily illustrating an IPS activematrix type liquid crystal display according to Example 1 of a firstembodiment of the invention;

FIGS. 5A and 5B are exemplary diagrams for explaining the functions ofoptical compensators;

FIGS. 6A and 6B are viewing angle characteristic charts for the contrastand chromaticity according to Example 1 of the first embodiment;

FIG. 7 is a contrast characteristic diagram with retardation as aparameter;

FIG. 8 is another contrast characteristic diagram with retardation as aparameter;

FIG. 9 is a viewing angle characteristic chart for the contrastaccording to Example 2 of the first embodiment;

FIGS. 10A and 10B are diagrams exemplarily showing an IPS active matrixtype liquid crystal display according to Example 1 of a secondembodiment of the invention;

FIG. 11 is a viewing angle characteristic chart for the contrastaccording to Example 1 of the second embodiment;

FIGS. 12A and 12B are viewing angle characteristic charts for thecontrast and chromaticity according to Example 2 of the secondembodiment;

FIGS. 13A and 13B are diagrams exemplarily illustrating the structure ofan IPS active matrix type liquid crystal display according to Example 1of a third embodiment of the invention;

FIG. 14 is a viewing angle characteristic chart for the contrastaccording to Example 1 of the third embodiment;

FIGS. 15A and 15B are viewing angle characteristic charts for thecontrast and chromaticity according to Example 1 of the thirdembodiment;

FIGS. 16A and 16B are diagrams exemplarily showing an IPS active matrixtype liquid crystal display according to Example 2 of the thirdembodiment;

FIG. 17 is a viewing angle characteristic chart for the contrastaccording to Example 2 of the third embodiment;

FIGS. 18A and 18B are viewing angle characteristic charts for thecontrast and chromaticity according to Example 2 of the thirdembodiment;

FIG. 19 is a contrast characteristic diagram with retardation as aparameter;

FIG. 20 is another contrast characteristic diagram with retardation as aparameter;

FIGS. 21A and 21B are diagrams exemplary illustrating an IPS activematrix type liquid crystal display according to Example 1 of a fourthembodiment of the invention;

FIG. 22 is a viewing angle characteristic chart for the contrastaccording to Example 1 of the fourth embodiment; and

FIGS. 23A and 23B are viewing angle characteristic charts for thecontrast and chromaticity according to Example 2 of the fourthembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will be described below withreference to the accompanying drawings.

First Embodiment Example 1 of First Embodiment

FIGS. 4A and 4B illustrate the structure of an in-plane switching type(IPS) active matrix type liquid crystal display 1 according to Example 1of the first embodiment of the invention. FIG. 4A is an exemplarydiagram showing the lamination structure of the liquid crystal display 1and FIG. 4B is a diagram exemplarily showing the alignment directionsand optical axes of individual layers. As shown in the diagrams, theliquid crystal display 1 has a liquid crystal display (LCD) panel havingan active device substrate 11, an opposing substrate 12 and a liquidcrystal layer 13 held sandwiched between the active device substrate 11and the opposing substrate 12. Provided on outside of the LCD panel 10on the opposing substrate side is an optical compensator group 20 whichhas first to third optical compensators 21 to 23 arranged outward frominside in order. A first polarizer 31 is laid out outside the opticalcompensator group 20. A fourth optical compensator 24 is provided onoutside of the LCD panel 10 on the active device substrate side and asecond polarizer 32 is arranged outside the fourth optical compensator24.

In the following description, the up and down direction and the left andright direction of the screen in case where an observer out of thediagrams makes a frontward observation of the liquid crystal displaywill be called “vertical direction” and “horizontal direction”,respectively. Further, the direction in which the opposing substrate 12is located and the direction in which the active device substrate 11 islocated will be called “top side” and “bottom side”, respectively, withrespect to the observer observing the LCD panel 10.

Referring to FIG. 4B, the liquid crystal of the liquid crystal layer 13is aligned nearly parallel to both substrates 11 and 12, and theretardation, Δ·d, of the liquid crystal layer 13 is 310 nm. With thealignment direction of the liquid crystal layer 13 being the verticaldirection of the screen, the direction of the absorption axis of thefirst polarizer 31 (the optical axis in a direction perpendicular to thedirection in which polarized light passes) is set perpendicular to thealignment direction of the liquid crystal layer 13. The direction of theabsorption axis of the second polarizer 32 is set parallel to thealignment direction of the liquid crystal layer 13.

The direction of the refractive index nx of the first opticalcompensator 21 is set parallel to the alignment direction of the liquidcrystal layer 13. The direction of the refractive index nx of the secondoptical compensator 22 is set perpendicular to the alignment directionof the liquid crystal layer 13. The direction of the refractive index nxof the third optical compensator 23 is set parallel to the direction ofthe absorption axis of the first polarizer 31. The direction of therefractive index nx of the fourth optical compensator 24 is set parallelto the direction of the absorption axis of the second polarizer 32.

Each of the optical compensators 21 to 24 has refractive indexes nx andny in the x direction and y direction in a plane, and a refractive indexnz in a thickness direction. Further, d is the film thickness of each ofthe optical compensators 21 to 24 in a direction perpendicular to thescreen. As the characteristics of the first optical compensator 21, theretardation (nx−ny)d was set equal to −380 nm and (nx−nz)/(nx−ny) wasset equal to 1.05. As the characteristics of the second opticalcompensator 22, the retardation (nx−ny)d was set equal to 314 nm and(nx−nz)/(nx−ny) was set equal to 0.538. For both of the third opticalcompensator 23 and the fourth optical compensator 24, the retardation(nx−ny)d was set equal to −6 nm and (nx−nz)/(nx−ny) was set equal to8.3.

When one observes the liquid crystal display 1 with the structure shownin FIGS. 4A and 4B from the front side, with no electric field appliedto the liquid crystal layer 13, the optical axes (alignment directionsand adsorption axes) of all of the polarizers 31 and 32, the opticalcompensators 21 to 24 and the liquid crystal layer 13 are positionedparallel or perpendicular. Therefore, light polarized by the secondpolarizer 32 reaches the first polarizer 31 without being influenced atall and the polarization plane of the polarized light coincides with thedirection of the absorption axis of the first polarizer 31, thus makinga dark state. The state in which no electric field is applied to theliquid crystal layer 13 is called a “black display state”.

As mentioned earlier, a polarizer comprises a polarization layer formedof a material having a polarization property and a protection layerwhich protects the polarization layer, and it is known that triacetylcellulose which is generally used to form the protection layer has anoptical anisotropy during the fabrication process of the polarizer. Theoptical anisotropy causes birefringence with respect to the light thatpasses the liquid crystal display at the time the viewing angle of theliquid crystal display is changed, thereby degrading the viewing anglecharacteristic. Such degradation increases the luminance in a dark statein case of conducting oblique observation, thus lowering the contrast.The third optical compensator 23 and the fourth optical compensator 24are provided with such characteristics as to respectively compensate forthe optical anisotropies of the first polarizer 31 and the secondpolarizer 32 respectively adjoining the optical compensators 23 and 24,thereby eliminating the influences of the optical anisotropy of eachpolarizer on the liquid crystal display 1.

The first optical compensator 21 suppresses the occurrence of thebirefringence effect on the LCD panel 10 in a black display state, whichis originated from the apparent deviation between the alignmentdirection and the polarization plane that is caused by obliqueobservation of the liquid crystal layer 13. The second opticalcompensator 22 demonstrates an compensation effect such that theapparent polarization plane of light having passed the 20, secondpolarizer 32 in case of oblique observation is always made parallel tothe absorption axis of the first polarizer 31, regardless of thedirection of the field of view. FIG. 5A exemplarily shows this effect.The effect suppresses light leakage low, no matter in whicheverdirection of the field of view an observation is made, so that a changein viewing angle does not result in lower contrast. Further, when theviewing angle is changed, the optical path of transmitted light becomeslonger as shown in FIG. 5B, so that the apparent retardation of theliquid crystal layer material or the like becomes greater. As theviewing angle changes, therefore, the wavelength of light which passesthrough the liquid crystal display 1 changes so that the colors on thescreen look changed. The individual optical compensators 21 to 24intervened in the invention can be constructed in such a way as tocompensate for a change in the wavelength of the passing light byadjusting their characteristics. This can also suppress color shiftingof the screen when the viewing angle is changed in the black displaystate.

FIG. 6A shows the results of measuring the viewing angle characteristicof the liquid crystal display having the structure in FIGS. 4A and 4Bwith EZcontrast, a product of ELDIM Company. In the diagram, thecontrast ratios are 800, 400, 200, 100, 50, 20, 10, 5 in order from thecenter. With regard to the viewing angle, the center is the front viewfield, and concentric circles have viewing angles of 20°, 40°, 60° and80° in order from the center side. The same is applied to similardiagrams. As apparent from FIG. 6A, it was confirmed that the liquidcrystal display having the structure in FIGS. 4A and 4B had a contrastof 100 or higher regardless of the viewing angle. The contrast ratio onthe front side was equivalent to the result of a measurement made on thestructure of the prior art. FIG. 6B shows the results of similarlymeasuring the viewing angle characteristic for the chromaticity at thetime of displaying black. It was also confirmed from the diagram that achange in chromaticity at the time of having changed the viewing anglewas suppressed, thus having suppressed color shifting low. Apparently,the invention can provide an IPS liquid crystal display which improvesthe contrast ratio in case of making oblique observation withoutlowering the contrast as obtained when making frontward observation, andprevents color shifting at the time of displaying black.

Studies have been made on the refractive index structures of opticalcompensators that provide good viewing angle characteristics in casewhere four optical compensators are used as mentioned above. First, forthe optical compensator corresponding to the first optical compensator21, the in-plane retardation (nx−ny)d and a parameter (nx−nz)/(nx−ny)indicating the degree of alignment in the thickness direction wererespectively set equal to −310 nm and 1.0 in such a way as to compensatefor the retardation of the liquid crystal layer 13. At this time, theparameters of the optical compensator corresponding to the secondoptical compensator 22 were changed and the lowest contrast at a polarangle of 80° was measured. The results of the measurement are shown inFIG. 7. It is apparent from the diagram that the lowest contrast in anoblique field of view would become equal to or greater than 20, showingthe effect of the invention, by setting the retardation (nx−ny)d of thesecond optical compensator 22 in a range of 160 nm to 370 nm and(nx−nz)/(nx−ny) in a range of 0.4 to 0.8.

Further, a similar measurement was made while changing the parameters ofthe first optical compensator 21 with the parameters of the secondoptical compensator 22 set to (nx−ny)d=270 nm and (nx−nz)/(nx−ny)=0.6which would show good characteristics in FIG. 7. The results of themeasurement are shown in FIG. 8. It is apparent from the diagram thatthe lowest contrast in an oblique field of view would become equal to orgreater than 20, showing the effect of the invention, by setting theretardation (nx−ny)d of the first optical compensator 21 in a range of−100 nm to −500 nm and (nx−nz)/(nx−ny) in a range of 0.7 to 1.2.

Example 2 of First Embodiment

Example 2 of the first embodiment, like Example 1 of the firstembodiment, takes a structure similar to the structure of the IPS activematrix type liquid crystal display shown in FIGS. 4A and 4B. As thecharacteristics of the first optical compensator 21, the retardation(nx−ny)d was set equal to −350 nm and (nx−nz)/(nx−ny) was set equal to1.14. As the characteristics of the second optical compensator 22, theretardation (nx−ny)d was set equal to 274 nm and (nx−nz)/(nx−ny) was setequal to 0.471. For both of the third optical compensator 23 and thefourth optical compensator 24, the retardation (nx−ny)d was set equal to−6 nm and (nx−nz)/(nx−ny) was set equal to 8.3.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display show that thecontrast becomes equal to or greater than approximately 200 from anyangle of observation as apparent from FIG. 9 and the contrast in casewhere observation is made from an oblique view field. The contrast ratioon the front side was equivalent to the result of the measurement madeon the structure of the prior art.

According to the first embodiment described above, it is preferable thatboth of the third optical compensator 23 and the fourth opticalcompensator 24 should have the retardation (nx−ny)d<0 nm and(nx−nz)/(nx−ny)>8.0 as the refractive index structures of the opticalcompensators that provide good viewing angle characteristics.Alternatively, it is preferable that the third optical compensator 23and the fourth optical compensator 24 should respectively have(nx−ny)d=0 nm and (nx−ny)d<0 nm. It became apparent that the thirdoptical compensator 23 and the fourth optical compensator 24 shouldpreferably have (nx−ny)d 0 nm and (nx−ny)d<−30 nm, respectively.

Second Embodiment Example 1 of Second Embodiment

FIGS. 10A and 10B show the structure of a liquid crystal display 1Aaccording to Example 1 of the second embodiment. FIG. 10A is anexemplary diagram showing a lamination structure and FIG. 10B is anexemplary diagram showing the alignment directions and optical axes ofindividual layers. In those diagrams, same reference symbols are givento those portions which are equivalent to the corresponding portions ofthe first embodiment. The liquid crystal display 1A comprises the LCDpanel 10, a first optical compensator group 40A laid out on the top sideof the LCD panel 10, the first polarizer 31 laid out on the top side ofthe first optical compensator group 40A, a second optical compensatorgroup 40B laid out on the bottom side of the LCD panel 10, and thesecond polarizer 32 laid out on the bottom side of the second opticalcompensator group 40B.

The first optical compensator group 40A comprises a first opticalcompensator 41 laid out on the top side of the LCD panel 10 and a secondoptical compensator 42 laid out on the top side of the first opticalcompensator 41. The second optical compensator group 40B comprises athird optical compensator 43 laid out on the bottom side of the LCDpanel 10 and a fourth optical compensator 44 laid out on the bottom sideof the third optical compensator 43.

The direction of the absorption axis of the first polarizer 31 is setperpendicular to the alignment direction of the liquid crystal layer 13.The direction of the absorption axis of the second polarizer 32 is setparallel to the alignment direction of the liquid crystal layer 13.

As the characteristics of the first optical compensator 41, theretardation (nx−ny)d was set equal to 274 nm and (nx−nz)/(nx−ny) was setequal to 0.471 and the direction of the refractive index nx was setperpendicular to the alignment direction of the liquid crystal layer 13.As the characteristics of the second optical compensator 42, theretardation (nx−ny)d was set equal to −6 nm and (nx−nz)/(nx−ny) was setequal to 8.3 and the direction of the refractive index nx was setparallel to the direction of the absorption axis of the first polarizer31. As the characteristics of the third optical compensator 43, theretardation (nx−ny)d was set equal to −350 nm and (nx−nz)/(nx−ny) wasset equal to 1.14 and the direction of the refractive index nx was setparallel to the alignment direction of the liquid crystal layer 13. Asthe characteristics of the fourth optical compensator 44, theretardation (nx−ny)d was set equal to −6 nm and (nx−nz)/(nx−ny) was setequal to 8.3 and the direction of the refractive index nx was setparallel to the direction of the absorption axis of the second polarizer32.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display 1A show that thecontrast becomes equal to or greater than 50 from any angle ofobservation as apparent from FIG. 11 and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art.

Example 2 of Second Embodiment

Example 2 of the second embodiment, like Example 1 of the secondembodiment, takes a structure similar to the structure of the IPS activematrix type liquid crystal display shown in FIGS. 10A and 10B. As thecharacteristics of the first optical compensator 41, the retardation(nx−ny)d was set equal to 314 nm and (nx−nz)/(nx−ny) was set equal to0.538 and the direction of the refractive index nx was set perpendicularto the alignment direction of the liquid crystal layer 13. As thecharacteristics of the second optical compensator 42, the retardation(nx−ny)d was set equal to −6 nm and (nx−nz)/(nx−ny) was set equal to 8.3and the direction of the refractive index nx was set parallel to thedirection of the absorption axis of the first polarizer 31. As thecharacteristics of the third optical compensator 43, the retardation(nx−ny)d was set equal to −380 nm and (nx−nz)/(nx−ny) was set equal to1.05 and the direction of the refractive index nx was set parallel tothe alignment direction of the liquid crystal layer 13. As thecharacteristics of the fourth optical compensator 44, the retardation(nx−ny)d was set equal to −6 nm and (nx−nz)/(nx−ny) was set equal to 8.3and the direction of the refractive index nx was set parallel to thedirection of the absorption axis of the second polarizer 32.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display show that thecontrast becomes equal to or greater than 20, irrespective of theviewing angle, as apparent from FIG. 12A and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art. FIG. 12B shows the results of similarlymeasuring the viewing angle characteristic for the chromaticity at thetime of displaying black. It is seen from those diagrams that a changein chromaticity at the time of changing the viewing angle has beensuppressed and color shifting has been suppressed low.

It is preferable that the second optical compensator 42 and the fourthoptical compensator 44 should have (nx−ny)d<0 nm and (nx−nz)/(nx−ny)>8.0as the refractive index structures of Examples 1 and 2 of the secondembodiment. Alternatively, it is preferable that the second opticalcompensator 42 and the fourth optical compensator 44 should respectivelyhave (nx−ny)d=0 nm and (nx−ny)d<0 nm. It became apparent that the secondoptical compensator 42 and the fourth optical compensator 44 shouldpreferably have (nx−ny)d=0 nm and (nx−ny)d<−30 nm, respectively.

Although four optical compensators are used in the first and secondembodiments, the functions of plural optical compensators may beintegrated into the function of a single optical compensator or thefunction of a single optical compensator may be separated into aplurality of functions of plural optical compensators. From theviewpoint of fabrication of optical compensators, lamination of multiplefilms is apt to lower the yield and it is preferable that the number offilms to be laminated should be three or less. In this respect, studieshave been made on the conditions that provide good viewing anglecharacteristics with two optical compensators. Examples that satisfy theconditions will be discussed below as the third and fourth embodiments.

Third Embodiment Example 1-1 of Third Embodiment

Exemplary diagrams of FIGS. 13A and 13B show the structure of a liquidcrystal display 1B according to Example 1-1 of the third embodiment. Theliquid crystal display 1B according to the embodiment comprises the LCDpanel 10, a first optical compensator group 50 laid out on the top sideof the LCD panel 10, the first polarizer 31 laid out on the top side ofthe first optical compensator group 50, and the second polarizer 32 laidout on the bottom side of the LCD panel 10.

The first optical compensator group 50 comprises a first opticalcompensator 51 laid out on the top side of the LCD panel 10 and a secondoptical compensator 52 laid out on the top side of the first opticalcompensator 51.

The direction of the absorption axis of the first polarizer 31 is setperpendicular to the alignment direction of the liquid crystal layer 13.The direction of the absorption axis of the second polarizer 32 is setparallel to the alignment direction of the liquid crystal layer 13.

As the characteristics of the first optical compensator 51, theretardation (nx−ny)d was set equal to −320 nm and (nx−nz)/(nx−ny) wasset equal to 1.00 and the direction of the refractive index nx was setparallel to the alignment direction of the liquid crystal layer 13. Asthe characteristics of the second optical compensator 52, theretardation (nx−ny)d was set equal to 412 nm and (nx−nz)/(nx−ny) was setequal to 0.774 and the direction of the refractive index nx was setperpendicular to the alignment direction of the liquid crystal layer 13.

When one observes the liquid crystal display 1B with the above-describedstructure from the front side, with no electric field applied to theliquid crystal layer 13, the optical axes of all of the polarizers 31and 32, the optical compensators 51 and 52 and the liquid crystal layer13 are positioned parallel or perpendicular. Therefore, light polarizedby the second polarizer 32 reaches the first polarizer 31 without beinginfluenced at all and the polarization plane of the polarized lightcoincides with the direction of the absorption axis of the firstpolarizer 31, thus making a dark state. As the optical anisotropy of thepolarizer protection layer and the apparent axial angle vary, however,light leaks and the wavelength of passing light varies in the obliquedirection, resulting in contrast reduction and coloring. The first andsecond optical compensators 51 and 52 together serve to compensate forlight leakage caused by such factors. The first optical compensator 51mainly has a birefringence effect in the liquid crystal layer 13 whilethe second optical compensator 52 has a main effect of compensating forthe axial angles of the polarizers 31 and 32.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display 1B show that thecontrast becomes equal to or greater than 20 from any angle ofobservation as apparent from FIG. 14 and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art.

Example 1-2 of Third Embodiment

Example 1-2 of the third embodiment, like Example 1-1 of the thirdembodiment, takes a structure similar to the structure of the IPS activematrix type liquid crystal display shown in FIGS. 13A and 13B. As thecharacteristics of the first optical compensator 51, the retardation(nx−ny)d was set equal to −186 nm and (nx−nz)/(nx−ny) was set equal to1.14 and the direction of the refractive index nx was set parallel tothe alignment direction of the liquid crystal layer 13. As thecharacteristics of the second optical compensator 52, the retardation(nx−ny)d was set equal to 402 nm and (nx−nz)/(nx−ny) was set equal to0.537 and the direction of the refractive index nx was set perpendicularto the alignment direction of the liquid crystal layer 13.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display show that thecontrast becomes equal to or greater than 5, irrespective of the viewingangle, as apparent from FIG. 15A and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art. FIG. 15B shows the results of similarlymeasuring the viewing angle characteristic for the chromaticity at thetime of displaying black. It is seen from those diagrams that a changein chromaticity at the time of changing the viewing angle has beensuppressed and color shifting has been suppressed low.

Example 1-3 of Third Embodiment

Example 1-3 of the third embodiment takes a structure similar to thestructure of the IPS active matrix type liquid crystal displayillustrated in Examples 1-1 and 1-2 of the third embodiment. As thecharacteristics of the first optical compensator 51, the retardation(nx−ny)d was set equal to −186 nm and (nx−nz)/(nx−ny) was set equal to1.3 and the direction of the refractive index nx was set parallel to thealignment direction of the liquid crystal layer 13. As thecharacteristics of the second optical compensator 52, the retardation(nx−ny)d was set equal to 402 nm and (nx−nz)/(nx−ny) was set equal to0.7 and the direction of the refractive index nx was set perpendicularto the alignment direction of the liquid crystal layer 13.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display show that thecontrast becomes equal to or greater than 5, irrespective of the viewingangle, and the contrast in case where observation is made from anoblique view field. The contrast ratio on the front side was equivalentto the result of the measurement made on the structure of the prior art.The results of similarly measuring the viewing angle characteristic forthe chromaticity at the time of displaying black show that a change inchromaticity at the time of changing the viewing angle has beensuppressed and color shifting has been suppressed low.

Example 1-4 of Third Embodiment

Example 1-4 of the third embodiment takes a structure similar to thestructure of the IPS active matrix type liquid crystal displayillustrated in Examples 1-1, 1-2 and 1-3 of the third embodiment. As thecharacteristics of the first optical compensator 51, the retardation(nx−ny)d was set equal to −186 nm and (nx−nz)/(nx−ny) was set equal to1.3 and the direction of the refractive index nx was set parallel to thealignment direction of the liquid crystal layer 13. As thecharacteristics of the second optical compensator 52, the retardation(nx−ny)d was set equal to 402 nm and (nx−nz)/(nx−ny) was set equal to0.54 and the direction of the refractive index nx was set perpendicularto the alignment direction of the liquid crystal layer 13.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display show that thecontrast becomes equal to or greater than 5, regardless of the viewingangle, and the contrast in case where observation is made from anoblique view field. The contrast ratio on the front side was equivalentto the result of the measurement made on the structure of, the priorart. The results of similarly measuring the viewing angle characteristicfor the chromaticity at the time of displaying black show that a changein chromaticity at the time of changing the viewing angle has beensuppressed and color shifting has been suppressed low.

Example 2-1 of Third Embodiment

FIGS. 13A and 13B are exemplary diagrams showing a liquid crystaldisplay 1C according to Example 2-1 of the third embodiment. The liquidcrystal display 1C according to Example 2 of the third embodiment,unlike Example 1 of the third embodiment, comprises the LCD panel 10,the first polarizer 31 laid out on the top side of the LCD panel 10, anoptical compensator group 60 laid out on the bottom side of the LCDpanel 10, and the second polarizer 32 laid out on the bottom side of theoptical compensator group 60. The optical compensator group 60 comprisesa first optical compensator 61 laid out on the bottom side of the LCDpanel 10 and a second optical compensator 62 laid out on the bottom sideof the first optical compensator 61.

The direction of the absorption axis of the first polarizer 31 is setparallel to the alignment direction of the liquid crystal layer 13. Thedirection of the absorption axis of the second polarizer 32 is setperpendicular to the alignment direction of the liquid crystal layer 13.

As the characteristics of the first optical compensator 61, theretardation (nx−ny)d was set equal to −320 nm and (nx−nz)/(nx−ny) wasset equal to 1.00 and the direction of the refractive index nx was setparallel to the alignment direction of the liquid crystal layer 13. Asthe characteristics of the second optical compensator 62, theretardation (nx−ny)d was set equal to 412 nm and (nx−nz)/(nx−ny) was setequal to 0.774 and the direction of the refractive index nx was setperpendicular to the alignment direction of the liquid crystal layer 13.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display 1C show that thecontrast becomes equal to or greater than 20 from any angle ofobservation as apparent from FIG. 17 and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art.

Example 2-2 of Third Embodiment

Example 2-2 of the third embodiment takes a structure similar to thestructure of the IPS active matrix type liquid crystal displayillustrated in Example 2-1 of the third embodiment. As thecharacteristics of the first optical compensator 61, the retardation(nx−ny)d was set equal to −186 nm and (nx−nz)/(nx−ny) was set equal to1.14 and the direction of the refractive index nx was set parallel tothe alignment direction of the liquid crystal layer 13. As thecharacteristics of the second optical compensator 62, the retardation(nx−ny)d was set equal to 402 nm and (nx−nz)/(nx−ny) was set equal to0.537 and the direction of the refractive index nx was set perpendicularto the alignment direction of the liquid crystal layer 13.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display show that thecontrast becomes equal to or greater than 5, irrespective of the viewingangle, as apparent from FIG. 18A and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art. FIG. 18B shows the results of similarlymeasuring the viewing angle characteristic for the chromaticity at thetime of displaying black. It is seen from those diagrams that a changein chromaticity at the time of changing the viewing angle has beensuppressed and color shifting has been suppressed low.

Studies have been made on the refractive index structures of opticalcompensators that provide good viewing angle characteristics in case ofthe second embodiment which uses two optical compensators as mentionedabove by using a scheme similar to the one discussed earlier. Thein-plane retardations (nx−ny)d and parameters Nz (=(nx−nz)/(nx−ny)) eachindicating the degree of alignment in the thickness direction of theindividual optical compensators were changed and were combined, and thecontrast and coloring were evaluated. As shown in FIGS. 19 and 20showing the evaluation results, it is apparent that the lowest contrastin an oblique field of view becomes equal to or greater than 5, showingthe effect of the invention, by respectively setting the retardation(nx−ny)d and (nx−nz)/(nx−ny) of one optical compensator in a range of250 nm to 450 nm and in a range of 0.4 to 1.3, and respectively settingthe retardation (nx−ny)d and (nx−nz)/(nx−ny) of the other opticalcompensator in a range of −150 nm to −500 nm and in a range of 0.7 to1.5.

Fourth Embodiment Example 1 of Fourth Embodiment

FIGS. 21A and 21B are exemplary diagrams showing a liquid crystaldisplay 1D according to Example 1 of the fourth embodiment. The liquidcrystal display 1D according to Example 1 of the fourth embodimentcomprises the LCD panel 10, a first optical compensator 71 laid out onthe top side of the LCD panel 10, the first polarizer 31 laid out on thetop side of the first optical compensator 71, a second opticalcompensator 72 laid out on the bottom side of the LCD panel 10, and thesecond polarizer 32 laid out on the bottom side of the second opticalcompensator 72.

The direction of the absorption axis of the first polarizer 31 is setperpendicular to the alignment direction of the liquid crystal layer 13.The direction of the absorption axis of the second polarizer 32 is setparallel to the alignment direction of the liquid crystal layer 13.

As the characteristics of the first optical compensator 71, theretardation (nx−ny)d was set equal to 412 nm and (nx−nz)/(nx−ny) was setequal to 0.774 and the direction of the refractive index nx was setperpendicular to the alignment direction of the liquid crystal layer 13.As the characteristics of the second optical compensator 72, theretardation (nx−ny)d was set equal to −320 nm and (nx−nz)/(nx−ny) wasset equal to 1.00 and the direction of the refractive index nx was setparallel to the alignment direction of the liquid crystal layer 13.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display 1D show that thecontrast becomes equal to or greater than 10 from any angle ofobservation as apparent from FIG. 22 and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art.

Example 2 of Fourth Embodiment

Example 2 of the fourth embodiment takes a structure similar to thestructure of the IPS active matrix type liquid crystal displayillustrated in Example 1 of the fourth embodiment. As thecharacteristics of the first optical compensator 71, the retardation(nx−ny)d was set equal to 402 nm and (nx−nz)/(nx−ny) was set equal to0.537 and the direction of the refractive index nx was set perpendicularto the alignment direction of the liquid crystal layer 13. As thecharacteristics of the second optical compensator 72, the retardation(nx−ny)d was set equal to −186 nm and (nx−nz)/(nx−ny) was set equal to1.14 and the direction of the refractive index nx was set parallel tothe alignment direction of the liquid crystal layer 13.

The results of measuring the viewing angle characteristic for thecontrast of the thus provided liquid crystal display show that thecontrast becomes equal to or greater than 5, regardless of the viewingangle, as apparent from FIG. 23A and the contrast in case whereobservation is made from an oblique view field. The contrast ratio onthe front side was equivalent to the result of the measurement made onthe structure of the prior art. FIG. 23B shows the results of similarlymeasuring the viewing angle characteristic for the chromaticity at thetime of displaying black. It is seen from those diagrams that a changein chromaticity at the time of changing the viewing angle has beensuppressed and color shifting has been suppressed low.

Although the number of optical compensators is fixed in the foregoingdescription of the individual embodiments, the effects of plural opticalcompensators may be integrated into the effect of a single opticalcompensator, or a single optical compensator may be separated into aplurality of optical compensators in order to demonstrate the optimaleffect.

Although the optical axes of the polarizers and optical compensators arefixed to specific directions in the illustrated structures, each opticalaxis can be set to any direction in accordance with the characteristicsand layout positions of the optical compensators, as long as it isparallel to or perpendicular to the alignment direction of the liquidcrystal.

According to the invention, as described above, an IPS active matrixtype liquid crystal display having an IPS LCD panel and first and secondpolarizers sandwiching the LCD panel has a single optical compensator orplural optical compensators laid out between the LCD panel and one ofboth polarizers or between the LCD panel and both polarizers. As thoseoptical compensators, an optical compensator which compensates forretardation of the liquid crystal layer and an optical compensator whichcompensates for retardation of the polarizers are provided, blackstretching does not occur even when observation is made on the activematrix type liquid crystal display from any viewing angle, contrastreduction does not occur and color shifting does not occur at the timeof displaying black.

1.-43. (canceled)
 44. A liquid crystal display comprising: an in-planeswitching type liquid crystal display panel having an active devicesubstrate, an opposing substrate and a liquid crystal layer heldsandwiched between said active device substrate and said opposingsubstrate; a first polarizer provided on one side of said liquid crystaldisplay panel; a second polarizer provided on the other side of saidliquid crystal display panel; and an optical compensator provided ateither between said liquid crystal display panel and said firstpolarizer or between said liquid crystal display panel and said secondpolarizer, wherein said liquid crystal layer has positiverefractive-index anisotropy and is aligned in a direction parallel tosaid substrate, the absorption axes of said first and second polarizersare perpendicular to each other in a direction parallel to saidsubstrate, one of the absorption axes is placed parallel to thealignment direction of said liquid crystal layer, and said opticalcompensator has two-axis refractive-index anisotropy, and the opticalaxes thereof are parallel to the alignment direction of said liquidcrystal layer, perpendicular to the alignment direction of said liquidcrystal layer on the plane of said substrate and perpendicular to theplane of said substrate.
 45. A liquid crystal display according to claim44, wherein said optical compensator has two-axis refractive-indexanisotropy, and the optical axes thereof are parallel to the alignmentdirection of said liquid crystal layer, perpendicular to the alignmentdirection of said liquid crystal layer on the plane of said substrateand perpendicular to the plane of said substrate, and relationships ofnx>ny and nx>nz are established when it is assumed that the refractiveindexes on the optical axes are nx, ny and nz.